ADVANCED MATERIALS & PROCESSES | APRIL 2025 25 blades and vanes fail in the hot section of the engine along grain boundaries that are transverse to the direction of applied stress. For better creep and fracture resistance, columnar grains are achieved through directional solidi- fication. In this process, the thermal gradient is tightly controlled during casting to obtain columnar grains where the grain boundaries are aligned in the longitudinal direction of the part. Elimination of grain boundaries further improves creep and environmental resistance to help airfoils withstand engine operating temperatures above the alloy’s melting temperature. This is achieved through the single crystal casting process where the primary orientation of the single crystal is controlled through a grain selector that allows preferential growth of a single grain orientation. Figure 4 shows a schematic of the process employed to generate both directionally solidified and single crystal castings. MODELING AND SIMULATION Additional control of the crystallographic orientation of airfoils—to improve temperature capability and life—can be achieved through epitaxial growth using a seed. Solidification modeling has helped engineer solutions throughout the years and Howmet has developed extensive databases to run simulations that predict its processes. coating furnaces, ceramic core processing equipment, wax assembly cells, shell lines, process automation, and artificial intelligence (AI) systems for assisted inspection are a few examples of equipment that is built internally at Howmet to keep intellectual property secure. The company has dedicated resources to advance the use of AI in manu- facturing, from inspection-assisted techniques and equipment reliability predictions to process prediction, validation, and correction methods. Figure 2 shows an example of an automation cell used in a finishing process. GRAIN STRUCTURE CONTROL Precise control of grain structure and chemistry is critical for aerospace engine applications. Three main casting processes are employed across Howmet to produce components made of nickel- base superalloys as well as titanium and aluminum alloys. The resulting grain structure and applications within the engine are listed in Fig. 3. An equiaxed grain structure is produced in conventionally cast parts. Tight control of grain size and uniformity are important for performance as Fig. 2 — Automation cells at Howmet reduce variation and improve productivity and safety. Fig. 3 — Advanced investment casting processes at Howmet generate three main grain structures for complex jet engine components: from left, equiaxed, directionally solidified, and single crystal.
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